An intense thermospheric jet on Titan

TL;DR

This paper reports the first direct measurements of Titan's upper atmospheric winds, revealing a strong prograde jet extending into the thermosphere with speeds up to 340 m/s, indicating complex dynamical processes.

Contribution

It provides new direct wind measurements in Titan's thermosphere using ALMA, showing an unexpected extension of prograde winds and the presence of an equatorial jet at high altitudes.

Findings

Strong prograde winds extend into Titan's thermosphere.

Winds reach speeds of ~340 m/s at 1000 km altitude.

HNC molecule is confined to the thermosphere above 870 km.

Abstract

Winds in Titan's lower and middle atmosphere have been determined by a variety of techniques, including direct measurements from the Huygens Probe over 0-150 km, Doppler shifts of molecular spectral lines in the optical, thermal infrared and mm ranges, probing altogether the ~100-450 km altitude range, and inferences from thermal field over 10 mbar - 10 -3 mbar (i.e. ~100-500 km) and from central flashes in stellar occultation curves. These measurements predominantly indicated strong prograde winds, reaching maximum speeds of ~150-200 m/s in the upper stratosphere, with important latitudinal and seasonal variations. However, these observations provided incomplete atmospheric sounding; in particular, the wind regime in Titan's upper mesosphere and thermosphere (500- 1200 km) has remained unconstrained so far. Here we report direct wind measurements based on Doppler shifts of six molecular species observed with ALMA. We show that unlike expectations, strong prograde winds extend up to the thermosphere, with the circulation progressively turning into an equatorial jet regime as altitude increases, reaching ~340 m/s at 1000 km. We suggest that these winds may represent the dynamical response of forcing by waves launched at upper stratospheric/mesospheric levels and/or magnetospheric-ionospheric interaction. We also demonstrate that the HNC distribution is restricted to Titan's thermosphere above ~870 km altitude.